1. Field of the Invention
This invention relates to accelerometers having a liquid inertia mass.
2. Description of the Prior Art
Accelerometers used in seismic exploration may take the form of a cylindrical chamber, closed at each end by a piezo-electric wafer. A quantity of mercury fills most of the volume of the closed chamber and serves as a liquid inertia mass. In most orientations of the principal axis of such an accelerometer unit, the accelerometer output signal is affected by both vertical and horizontal accelerating forces. One or the other of the acceleration vectors can be isolated by additively or subtractively combining the respective electrical outputs of the two piezo-electric wafers. One such accelerometer is disclosed in U.S. Pat. No. 4,334,296, issued June 8, 1982 to E. M. Hall, Jr., which is incorporated herein by reference.
The degree of cancellation of an unwanted acceleration vector depends of course, on the precision with which the two wafers can be matched as to their electrical and mechanical parameters. It has been our experience that it is impractical from a commercial production standpoint to hand-pick unmounted wafers that match exactly. We have also found that wafers which match precisely before assembly, commonly become mismatched during the assembly process.
One method for matching the sensitivities of the two piezo-electric wafers after assembly is disclosed in U.S. Pat. No. 4,395,908 issued Aug. 8, 1983 to R. C. Shopland. Another method is disclosed in a co-pending U.S. patent application Ser. No. 517,899, now U.S. Pat. No. 4,455,500 in the name of C. H. Savit et al. Although those methods served their intended purpose of matching the mechanical and electrical parameters of the wafers of an assembled accelerometer, the methods tend to complicate the commercial manufacturing operation.